The “Slugs-test” for extrusion-based additive manufacturing: Protocol, analysis and practical limits

This paper introduces a novel rheological technique allowing for the assessment of printable materials yield stress at nozzle exit in the case of extrusion-based 3D printing. This technique is derived from the analysis of the specific gravity-induced non-Newtonian flow that takes place at nozzle exit, which is at the origin of the formation of material drops or so-called slugs. A simple connected balance located below the nozzle gives access to the slugs mass distribution, the average value and the variability of which allow for the computation of the yield stress and, in parallel, for the assessment of the material homogeneity. In this paper, this method is first experimentally validated on simple materials in simple extruders before it is applied to a real printing system. The equations allowing for the yield stress computation are derived. The accuracy of the technique and its range of applicability are discussed.